In recent years, the demand for high definition and narrow frames in light emitting display devices for mobile use is increasing. Display devices using a liquid crystal display device (LCD) or organic light-emitting diode (OLED) in a display part, or display devices such as electronic paper are being adopted as display devices for mobile use.
A display device using an organic EL element as described above does not require a backlight or polarization plate which are necessary in liquid crystal display devices. Furthermore, since a drive voltage of a light emitting element which is a light source is low, a display device using an organic EL element is attracting considerable attention as a low power consumption and thin light emitting display device. In addition, since a display device using an organic EL element is formed just with a thin film, it is possible to realize a display device which can be bent (flexible). Such a flexible display device does not use a glass substrate. Therefore, the flexible display device has attracted considerable attention since it is possible to realize a display device which is thin and does not break easily.
Luminance of an organic EL element is changed by a current flowing to the organic EL element. A current which flows to the organic EL element is affected by the characteristics of a thin film transistor (TFT) used in an active matrix panel. In an organic EL display device, a drive transistor is connected in series between a power supply line and an organic EL element. Therefore, the current which flows to the organic EL element is affected by variation in a threshold voltage (VTH) of the drive transistor. When the current which flows to an organic EL element is different in each pixel, display unevenness is occurred and it leads to a decrease in display quality.
Therefore, a VTH compensation circuit has been developed in order to suppress the effects on display quality by the variation in characteristics of a drive transistor. A VTH compensation is a technology for suppressing the variation in characteristics of a drive transistor by a constant current circuit for fixing a current flowing to an organic EL element.
For example, as is shown in Japanese Laid Open Patent Publication No. 2009-276744, a VTH compensation circuit can reduce the effects of a variation in characteristics of a drive transistor. Therefore, the amount of current, which is decided by input gradation data, supplied to an organic EL element is accurately controlled. Therefore, since VTH variation inherent in a drive transistor is effectively compensated, display quality of an organic EL display device is significantly improved.
However, a VTH compensation circuit requires to control a plurality of transistors which each of pixels has. Control circuits are arranged in a periphery region of the display device. Since not only signals for the transistors of the pixels of pixels but also a signal for the VTH compensation is needed, a driver circuit becomes large and thereby the area of the periphery region becomes large. As a result, the frame becomes wide.